Cross-platform way to determine if file has been edited?

devgirl05 0
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I am writing a cross-platform (big 3 - Linux, MAC, Windows) backup program, so I need to know if a file has been edited since last time. My plan is to save the last save time in a file and check the real situation of a folder against the data in the file to determine which files need to be backed up or updated.

I would like to avoid methods that require a lot of processing power (like diff, or counting bytes).

In this similar post, people suggested to use fstat(), but that solution would be a last resort for me because I was hoping for a cross-platform solution that can be solved with pure C. As far as I know, fstat is a (2), and in my man page it appears as (1), which (to my understanding) means that it is a system function in Linux and isn't a part of the standard C library. I have searched for fstat on windows, but could only find some android version.

Is there some other way to access file metadata? Is there some other solution to this? I am open to any suggestions and am ok if it sometimes false-flags, as long as it backs up data correctly and doesn't waste resources on backing up everything all the time.

Please help!

Thank you!

c
2 Answers

fstat is still the way to do this, but on Windows it's called _fstat. You can check for the _MSC_VER macro which will be defined if you're building with MSVC, and if so create a macro alias for fstat.

You can do the same for struct stat which MSVC calls struct _stat:

#ifdef _MSC_VER
#define fstat(fd,buf) _fstat(fd,buf)
typedef struct _stat stat_struct;
#else
typedef struct stat stat_struct;
#endif

Then you can use fstat and pass it an argument of type stat_struct for the second argument.

I have a decently sized cross platform open source application that uses this technique.

dbush0

My plan is to save the last save time in a file and check the real situation of a folder against the data in the file to determine which files need to be backed up or updated.

Ok.

I was hoping for a cross-platform solution that can be solved with pure C.

If by "pure C" you mean relying on only language features and library functions defined by the C language specification, then I'm afraid you're out of luck. Pure C (in that sense) has no concept of persistent file metadata such as modification timestamps. All functions and data structures dealing with such things are extensions or third-party libraries.

You can rely on standard POSIX facilities (such as fstat()) for both Linux and Mac, but Windows does not provide that. At least, Windows does not provide it exactly. The Microsoft C library does provide some POSIX compatibility functions, but it somewhat maddeningly uses modified names for them. In particular, it offers several flavors of _fstat() (note leading underscore). With a little bit of macro glue, it should not be too hard to make your program use POSIX fstat() on Linux and Mac, and use one of the _fstat() flavors on Windows.

John Bollinger0
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